Combinations of Highly Active Antiretroviral Therapy (HAART) have dramatically decreased morbidity and mortality in HIV-infected individuals. Despite prolonged treatment, infectious HIV continues to replicate and reside latently in reservoirs, creating a major obstacle in HIV eradication. The majority of HIV-infected individuals that undergo therapy interruption because of severe side effects, and/or high costs, rapidly develop an increase in viral loads and a decrease in CD4+ T-cell counts. This viral rebound during therapy interruption is not clearly associated v/ith "the major reservoir", resting CD4+ T-cells. The ability of HIV-infected monocytes to avoid cytopathology and the inability of HAART to block viral replication in these cells as efficiently as in CD4+ T cells may permit a continuous production of HIV in monocytes in patients under suppressive HAART. This suggests that monocytes could be important reservoirs and contributors to residual replication. We hypothesize that residual replication is an important factor in viral rebound after treatment interruption and that monocytes are a major contributor to residual replication. We propose to isolate and compare the proviral DNA of active CD4+ T cells, resting CD4+ T cells and HIV-infected monocytes and compare them phylogenetically with the residual replication during suppressive HAART to determine the contribution of monocytes and other types of cells in residual replication. We will isolate and measure this ongoing replication by quantifying the 2-LTR DNA episomal circles before therapy interruption and correlate them with the degree and rate of plasma virus RNA of the viral rebound and decrease of CD4+ T-cells during therapy interruption. We also propose to phylogenetically compare the C2-V3 env sequence of the 2-LTR circles before treatment interruption with the C2-V3 env sequence of the plasma virus RNA in viral rebound to determine if viral rebound is at least in part a result of residual replication. The determination of the viral source that emerges during HAART interruption would help design more effective therapies and strategies against HIV. The results obtained from this proposed work will be useful to better understand both the causes and the sources of the failure that is frequently observed during treatment interruption in patients with a previous clinical history of successful HAART. In addition, this study could lead to a method to systematically evaluate some of the risks and benefits of therapy interruption in the individual patient.